A magnetic force of attracting a magnetic particle to a magnet is represented by a product between a magnetic flux density (B) and a magnetization gradient (AB) at a position at which the magnetic particle is placed. A magnetic separation method of placing thin lines of a ferromagnet under a uniform magnetic field and generating a high magnetization gradient near the thin lines was proposed in the late 1960s and then evolved to high gradient magnetic separators in the United States. Nowadays, magnetic separators utilizing similar principles are sold by many magnetic separator manufacturers.
For example, a Jones-type wet high gradient magnetic separator is widely used as the magnetic separator. FIGS. 1A and 1B are depictive diagrams depicting overviews of a Jones-type wet high gradient magnetic separator.
As illustrated in FIG. 1A, a magnetic separator 100 includes as main members, a high gradient magnetic separating section 50 including an electromagnet 50a, a magnetic filter 50b, and a magnetic separation flow path 50c, a sorting target fluid introducing flow path 101b coupled to one end of the magnetic separation flow path 50c via an on-off valve 101a and capable of introducing a sorting target fluid into the magnetic separation flow path 50c, a non-magnetically attractable substance discharging flow path 103b coupled to the other end of the magnetic separation flow path 50c via an on-off valve 103a and capable of discharging from the magnetic separation flow path 50c, the sorting target fluid from which any magnetically-attractable substance has been magnetically attracted to the magnetic filter 50b, a carrier fluid introducing flow path 104b coupled to the other end of the magnetic separation flow path 50c via an on-off valve 104a and capable of introducing into the magnetic separation flow path 50c, a carrier fluid (e.g., water) capable of carrying the magnetically attractable substance detached from the magnetic filter 50b, and a magnetically attractable substance discharging flow path 105b coupled to the one end of the magnetic separation flow path 50c via an on-off valve 105a and capable of discharging from the magnetic separation flow path 50c, the carrier fluid carrying the magnetically attractable substance detached from the magnetic filter 50b. 
The magnetic separator 100 is configured to sort out the magnetically attractable substance and the non-magnetically attractable substance by separating them from the sorting target fluid.
First, as illustrated by arrows in FIG. 1A, only the on-off valve 101a of the on-off valves on the one end of the magnetic separation flow path 50c is opened to the magnetic separation flow path 50c in a state that the electromagnet 50a is excited, to introduce into the magnetic separation flow path 50c, the sorting target fluid introduced into the sorting target fluid introducing flow path 101b by means of a pump 101d from a storing section 101c storing the sorting target fluid and have the magnetically attractable substance magnetically attracted to the magnetic filter 50b, and only the on-off valve 103a of the on-off valves on the other end of the magnetic separation flow path 50c is opened to discharge the sorting target fluid from which the magnetically attractable substance has been magnetically attracted into the non-magnetically attractable substance discharging flow path 103b and recover the sorting target fluid into a non-magnetically attractable substance recovering section 103c (a non-magnetically attractable substance sorting step).
Next, as illustrated by arrows in FIG. 1B, only the on-off valve 104a of the on-off valves on the other end of the magnetic separation flow path 50c is opened to the magnetic separation flow path 50c in a state that the electromagnet 50a is released from excitation, to introduce the carrier fluid into the magnetic separation flow path 50c from the carrier fluid introducing flow path 104b, and only the on-off valve 105a of the on-off valves on the one end of the magnetic separation flow path 50c is opened to let the carrier fluid carry the magnetically attractable substance detached from the magnetic filter 50c to discharge the magnetically attractable substance from the magnetic separation flow path 50c into the magnetically attractable substance discharging flow path 105b and recover the magnetically attractable substance into a magnetically attractable substance recovering section 105c (a magnetically attractable substance sorting step).
A magnetic filter used in a magnetic separator is called matrix, and there are known matrices made of an expanded metal, steel wool, an iron ball, etc. (see PTL 1). Particularly, matrices made of an expanded metal and steel wool generate a locally-high magnetization gradient (AB), and are hence widely used for magnetically attracting magnetically attractable substances securely with a strong magnetic force.
The present applicant has previously invented and filed a patent application for a technique of sorting red, blue, and green phosphors from a mixture of phosphors color by color by means of a magnetic force with a high gradient magnetic separator (see PTL 2).
However, matrices made of an expanded metal, steel wool, etc., in which ferromagnet thin lines constituting the expanded metal, the steel wool, etc. are disposed in an intricately entwined state, have a problem that not only magnetic particles magnetically attractable to the ferromagnet thin lines but also many non-magnetic particles unintended for being magnetically attracted to the ferromagnet thin lines get entangled into the structure of the ferromagnet thin lines, leading to a poor sorting accuracy. Particularly, at a position at which a locally-high magnetization gradient (ΔB) is generated, magnetic particles magnetically attracted thereto earlier impede passage of non-magnetic particles to come later and block the flow path, leading to a large amount of non-magnetic particles entangled.
To overcome these problems, it is conceivable to dispose the ferromagnet thin lines sparsely. However, ferromagnet thin lines, which although can generate a locally-high magnetization gradient (ΔB), put many spatial regions under a low magnetization gradient (ΔB). Therefore, sparsely disposed ferromagnet thin lines restrict the area effective for magnetically attracting magnetic particles and allow magnetic particles to get through the ferromagnet thin lines via the spaces under the low magnetization gradient (ΔB), leading to the problem of the poor sorting accuracy.
To overcome these problems, it is necessary to figure out an appropriate magnetization gradient (ΔB) in the matrix space. However, matrices made of an expanded metal, steel wool, etc., in which the ferromagnet thin lines are disposed in the matrices in an irregular disposition, have a problem of rejecting previous recognition of a correct magnetic force distribution in the matrix space by simulation or the like.
Hence, magnetic separators using conventional matrices are operated in a manner that the matrix is used in a state that ferromagnet thin lines are disposed in an intricately entwined state, and magnetic particles are detached and recovered from the ferromagnet thin lines frequently before many non-magnetic particles are entangled into the ferromagnet thin lines. This entails a problem that an amount processable by one detaching/recovering operation is low, leading to a poor sorting efficiency.
Further, because of the incapability of previously recognizing a correct magnetic force distribution in the matrix space by simulation or the like, it is unknown until a matrix is actually prototyped and used for a sorting test whether the matrix has a good performance or not, which has disturbed development of a high-performance matrix.